1. Field of the Invention
The present invention relates to a lens array, a manufacturing method thereof, an LED head having the lens array, an exposure device having the LED head, an image forming apparatus having the exposure device, and a reading apparatus.
2. Description of Related Art
In an image forming apparatus such as an electrophotographic printer, a rod lens array consisting of multiple rod lenses is used in an exposure device having an LED array consisting of multiple LEDs (light emitting diodes) arranged in array. In a reading apparatus such as an image scanner, a facsimile machine, and the like, the rod lenses array is used in an optical system for forming an optical image of a read document at a light receiving unit having multiple light receiving elements arranged in array. The rod lens is formed by impregnating a glass fiber with ions so that the refraction index becomes lower from the center portion toward the peripheral portion, and is an optical element for forming a same-size erect image of an object. A lens array having multiple rod lenses arranged in array is an optical system for forming an optical image of an object on a line. As disclosed in Japanese Patent Application Publication No. 2003-202411, multiple micro lenses may be integrally formed through resin molding method to constitute an optical system for forming an optical image of an object on a line.
There exists a lens array having multiple micro lenses arranged in array to constitute an optical system for forming a same-size erect image of a document on a line on a light receiving unit consisting of multiple light receiving elements arranged in array. As described above, a high resolution can be obtained with the integrally-formed lens array including high-precision micro lenses efficiently manufactured though plastic injection molding.
In the optical system in which the micro lenses are arranged in array and the optical image of an object is formed on a line as disclosed in Japanese Patent Application Publication No. 2003-202411, the resolution relies on the shape of the micro lenses constituting the optical system. For example, the resolution of the optical image will not deteriorate in a case where high-precision micro lenses are used in which the central axes on the light emitting side are in line with the central axes on the object plane side in the micro lenses forming the optical system and lens pairs. However, the resolution of the optical image will decrease if low-precision micro lenses are used in which the central axes on the light emitting side are not in line with the central axes on the object plane side in the micro lenses forming the optical system.
In a case where the conventional lens array is elongated in an arrangement direction of the micro lenses to extend a printing area, the high-precision micro lenses cannot be manufactured, and the precision of each lens varies depending on a position in the longitudinal direction of the lens array. That is, there exists a problem that each micro lens has different values in the residual aberration, the transmission, the focal length, and the like due to errors of the lens shape and the refraction rate among multiple micro lenses arranged in a row.